One function of control systems applied to commercial refrigeration systems is to control cooling capacity in response to variations in refrigeration load. Often this involves on/off control of fixed speed compressors and/or variable control of variable speed compressors. When multiple compressors in a parallel arrangement are used to provide refrigeration to a plurality of evaporators operating at varying temperatures, suction pressure is generally used as a control variable input to the control system. Often a controller implementing a proportional-integral-derivative control algorithm processes a sensed suction pressure common to all the compressors in the parallel arrangement and determines a control output for one or more compressors to maintain cooling capacity at a level that closely matches the refrigeration load presented by the plurality of evaporators.
Suction pressure, being representative of temperature in the attached evaporator coil(s), is non-linear over the range of operating evaporator temperatures required in a typical commercial refrigeration system, and controllers implementing proportional-integral-derivative (“PID”) control algorithms do not operate efficiently on non-linear functions. Therefore, the use of a controller implementing a PID control algorithm in a commercial refrigeration system results in inefficient operation of the commercial refrigeration system or in additional cost for tuning of the PID control algorithm to specific operating parameters as required by the application.